1. Field of the Invention
The present invention relates to a method for protecting tuyeres for refining molten iron and is a novel proposal of results obtained by conducting a variety of studies and experiments with respect to a protecting fluid in the annulus of concentric tuyeres used for blowing a refining gas flow consisting of oxygen or a gas containing at least oxygen in a bottom-blown or a top-and-bottom-blown converter and the like.
2. Description of the Prior Art
As the protecting fluids of the double jet pipe tuyeres for refining molten iron, hydrocarbon gases used in oxygen bottom-blown converter usually known as OBM/Q-BOP, for example, propane, butane, natural gas and the like, or kerosene used in a bottom-blown converter usually known as LWS have been heretofore well known. These already known protecting fluids flow in such a state that these fluids surround the oxidizing gas, particularly pure oxygen gas constituting an axial core flow of the above described tuyere in a shrouding form, whereby it has been attempted to prolong the durable life of the tuyere owing to the cooling function of the shrouding gas. However, the above described protecting fluids contain hydrogen atom and a part of said hydrogen is absorbed in the molten iron and adversely affects upon the quality of the product.
In the bottom-blown converter for refining stainless steel usually known as AOD, protecting fluids containing no hydrogen atom, such as an inert gas, particularly argon gas or nitrogen gas are used, but these gases are not thermally decomposed at high temperatures, so that these gases do not show the heat removing effect which satisfactorily cools the tip of the tuyere opening at the molten iron bath side and the durable life of the tuyere is not greater than 350 heats and is inferior to the above described OBM/Q-BOP, which is greater than 1,000 heats.
Other than the above described protecting fluids, it has been already well known to use gaseous or liquid carbon dioxide as the protecting fluid containing no hydrogen. For example, gaseous carbon dioxide is disclosed in Japanese Pat. No. 447,093 (Japanese Patent Application Publication No. 24,183/1964). Liquid carbon dioxide is disclosed in Rev. Metallurgie (1978), P. 13.about.19. However, the cooling effect of carbon dioxide relies only upon the same small effect of removing heat as in argon gas or nitrogen gas, because no decomposition reaction is caused different from hydrocarbons and kerosene. If an explanation is made with reference to propane used as a hydrocarbon heretofore, it has been known from experiment that the tuyere can be satisfactorily protected by supplying about 4% by volume based on oxygen gas in the axial core flow of the tuyere. The effect of said propane for removing heat is attained by two factors. One factor is sensible heat variation when propane gas is raised from room temperature to 1,600.degree. C. which is the temperature of the molten iron bath and another factor is heat removal owing to endothermic reaction when propane, C.sub.3 H.sub.8 is decomposed at high temperature into C and H.sub.2. The sum of the above described two endothermic amounts calculated by the well known thermodynamic constant is about 78 Kcal/mol. While in the case of gaseous carbon dioxide, the decomposition reaction does not occur even if the heating is effected up to 1,600.degree. C., and the tuyere is cooled only by the variation of sensible heat amount when carbon dioxide at room temperature is heated to 1,600.degree. C. Therefore, an amount of heat removed by gaseous carbon dioxide is calculated to be 18.4 Kcal/mol. Similarly, the endothermic amount when liquid carbon dioxide is used, is 21.5 Kcal/mol when the calculation is effected by using the well known thermodynamic constant and this value is not greatly different from the above described value of gaseous carbon dioxide. Accordingly, in order to obtain the same cooling effect as in 4% by volume based on oxygen of propane by using carbon dioxide, carbon dioxide corresponding to 15.about.17% by volume based on oxygen of gaseous carbon dioxide is necessary. However, if such a large amount of carbon dioxide must be used, even if the problem of hydrogen pick-up which has been a defect in view of quality, is solved, such a means is not only more expensive than the already known propane, but also the heat balance in the converter is greatly worsened and it is difficult to obtain the same blow finishing temperature unless iron ore is reduced by 25 kg/ton molten steel as compared with the usual blowing. This means that cheap iron ore cannot be used as iron source and hence iron yield is lowered.
Furthermore, carbon dioxide is an oxidizing gas and therefore damages magnesia carbon bricks around the tuyeres and the durable life of the refractory is lowered.
As mentioned above, the idea in which carbon dioxide is used as the protecting fluid has been already proposed but this cannot compete with the conventional method using propane in view of economy, so that carbon dioxide has not been commercially practically used.
In Japanese Patent Application Publication No. 48,568/1980, there is disclosed that regarding the above described OBM process, carbon monoxide is usd as the protecting fluid other than the above described hydrocarbons, rare gas and carbon dioxide but the practical use is limited to the above described propane. Furthermore, Japanese Patent Laid-open Specification No. 93,814/1981 discloses that "the exhaust gas recovered from the bottom-blown refining furnace in the unburnt form" is used as the cooling medium but this prior art mainly aims the utilization of carbon dioxide in the exhaust gas and the cooling function of carbon monoxide which is the major part of volume of the exhaust gas, is neglected and rather there is mentioned that carbon monoxide is not desirable. In fact, the cooling action of carbon monoxide has not been taken into consideration.